1. Field of the Invention
This invention generally relates to radar techniques for determining angular location of a target and, more particularly, to an improvement in the monopulse technique so as to maintain accuracy of the monopulse ratio in the presence of simultaneous jamming from multiple mainlobe jammers.
2. Description of the Prior Art
The monopulse technique is a radar technique in which the angular location of a target can be determined within fractions of a beamwidth by comparing measurements received by two or more simultaneous beams. This technique for direction of arrival (DOA) estimation of a target is widely employed in modern surveillance and tracking radar. See, for example, D. K. Barton, "Modern Radar Systems Analysis", Artech House (1988), M. Sherman, "Monopulse Principles and Techniques", Artech House (1984),and I. Leanov and K. I. Fomichev, "Monopulse Radar", Artech House (1986). In a typical phased array or digital beamforming (DBF) radar, one beam is formed in transmission, and two beams are formed on reception for angle measurement.
The monopulse technique may be implemented for a linear array of N antenna elements which provide respective signals x(0), . . . , x(N-1) to the beam forming network from the elemental receivers. The output signals of the beam forming network are the sum, .SIGMA., and difference, .DELTA., signals which are processed in a processor to generate an output signal, .theta., representing the direction of arrival estimation.
In the beamforming network, each of the N input signals is split into two paths, linearly weighted, and then added together. The sum .SIGMA. and difference .DELTA. signals may be expressed in the form EQU .SIGMA.=W.sub..SIGMA..sup.H x (1) EQU .DELTA.=W.sub..DELTA..sup.H x (2)
respectively, where W.sub..SIGMA. is real and even weighting, W.sub..DELTA. is purely imaginary and odd weighting, H indicates the complex conjugate transpose and x is the vector of the measurements. When there is no jamming, Taylor and Bayliss weightings are typically used for sum beams and difference beams, respectively, so as to have a narrow mainlobe and low sidelobes. In the presence of jamming, the weights are adapted so as to form nulls responsive to the jammers. The quiescent Taylor and Bayliss weightings are designed for reducing the sidelobes in a practical system. See Y. T. Lo and S. W. Lee, "Antenna Handbook," Theory, Applications and Design, Van Nostrand Reinhold Company, New York (1988), Chapter 13.
In a typical antenna pattern, the mainlobe of the pattern is a central beam surrounded by minor lobes, commonly referred to as sidelobes. Typically, it is desired to have a narrow mainlobe, high gain and low sidelobes so that the desired target within the mainlobe is enhanced and the response to clutter and jamming outside the mainlobe is attenuated. The sidelobe levels of an antenna pattern can be described in several ways. The most common expression is the relative sidelobe level, defined as the peak level of the highest sidelobe relative to the peak level of the main beam. Sidelobe levels can also be quantified in terms of their absolute level relative to isotropic.
The term "monopulse" refers to the fact that the echo from a single transmitter pulse returning from a target is used to measure the angle of the target, and typically, one beam (instead of two beams) is formed in transmission, and two beams are formed on reception for angle measurement. The sum beam has a symmetrical amplitude profile with its maximum at the boresight, and the difference beam has an antisymmetrical amplitude profile with zero response at the boresight. The DOA of a target signal can be determined accurately through a look-up table by evaluating the monopulse ratio, i.e., the real part of .DELTA./.SIGMA.. In fact, for a noiseless case and for uniform weighting, the monopulse ratio is exactly as given by ##EQU1## where T=sin(.theta.) and (.theta.) is the desired DOA, d is the array element spacing, N is the number of sensor elements, and .lambda. is the wavelength. This equation enables T and the corresponding .theta. to be determined exactly. In the presence of noise, the development of the DOA maximum likelihood estimator also leads naturally to monopulse processing using sum and difference beams. See R. C. David, L. E. Brennan, and I. S. Reed, "Angle Estimation with Adaptive Arrays in External Noise Field," IEEE Trans. on Aerospace and Electronic Systems, Vol. AES-12, No. 2, March 1976. For zero-mean noise, the estimator is unbiased with mean square error (MSE) given by ##EQU2## which is known to be the monopulse sensitivity factor, f, is the monopulse function and f is the derivative of f. SNR is the signal-to-noise ratio at the elemental level,and g(T) is the two-way sum beam antenna pattern.
Various authors have defined the monopulse sensitivity factor in different ways (see R. R. Kinsey, "Monopulse Difference Slope and Gain Standards", IRE Trans, Vol AP-10, pp. 343-344, May 1962.) In this application, the monopulse sensitivity factor is defined as the constant of proportionality required in the denominator of the root-mean-square-error (RMSE) to convert the square root of twice the boresight signal-to-noise ratio in the beam to RMSE. Defined in this manner, the monopulse sensitivity factor has the desirable property of containing all target angle-of-arrival information. See D. J. Murrow, "Height Finding and 3D Radar," Chapter 20, Radar Handbook (2nd Edition), McGraw-Hill.
This technique can also be considered for a planar array where the target azimuth and elevation angles are desired. In this setup, a set of sum (.SIGMA..sub.e) and difference (.DELTA..sub.e) beams are formed along the elevation axis with input signals from each column of sensors. The .SIGMA..sub.e beams are then linearly combined in a row beamformer to form the sum (.SIGMA.=.SIGMA..sub.a .SIGMA..sub.e) and difference (.DELTA..sub.A =.DELTA..sub.a .SIGMA..sub.e) beam along the azimuth axis, where .SIGMA..sub.e and .DELTA..sub.a are the effective row sum beam and row difference beam respectively. Similarly, the .DELTA..sub.e beams are linearly combined in an adder to form the sum (.DELTA..sub.E =.SIGMA..sub.a .DELTA..sub.e) and difference (.DELTA..sub.A =.DELTA..sub.a .DELTA..sub.e) beams along the azimuth axis. Monopulse ratios along azimuth or elevation direction can then be formed giving the azimuth and elevation DOA estimates by using the following equations: ##EQU3## These derivations make use of the separable property of the planar array patterns.
The monopulse technique for DOA estimation fails when there is sidelobe jamming (SLJ) and/or mainlobe jamming (MLJ). If not effectively countered, electronic jamming prevents successful radar target detection and tracking. The situation is exacerbated by introduction of stealth technology to reduce the radar cross section (RCS) of unfriendly aircraft targets. The frequency dependence of the RCS encourages the use of lower microwave frequency bands for detection. This leads to large apertures to achieve angular resolution. Large apertures to achieve small beamwidth result in interception of more jamming. On the other hand, constrained apertures lead to wider beamwidth, which implies interception of more mainlobe jamming.
Heretofore, no viable or practical technique for cancelling multiple mainlobe jammers while preserving target detection and monopulse accuracy has been developed or fielded in a radar. Techniques developed for simultaneous nulling in the sum and difference channels of a monopulse radar work well in the presence of sidelobe jammers but not in the presence of mainlobe jammers. In a marine environment, the operation of a shipboard radar can be severely limited by the presence of multipath jamming (i.e. two or more jamming paths in close proximity to each other and the jamming are received within the main beam), particularly at low elevation angles. Moreover, the jammers are coherent as they originate from the same source. This makes the conception and development of a technique applicable to the multipath mainlobe jamming problem one of the more pressing and critical issues facing radar today. The challenge is to cancel multiple mainlobe jammers while maintaining target detection and angle estimation accuracy on mainlobe targets.